给定一个二叉树,返回其按层次遍历的节点值。 (即逐层地,从左到右访问所有节点)。
例如:
给定二叉树: [3,9,20,null,null,15,7]
,
3 / \ 9 20 / \ 15 7
返回其层次遍历结果:
[ [3], [9,20], [15,7] ]
先进先出
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def levelOrder(self, root):
"""
:type root: TreeNode
:rtype: List[List[int]]
"""
if root is None:
return []
stack = [root]
list_sum = []
while stack: # 设置none出
list = [] #出栈
for i in range(len(stack)): #每一层
cur = stack.pop(0)
list.append(cur.val)
if cur.left:
stack.append(cur.left) #入栈
if cur.right:
stack.append(cur.right)
list_sum.append(list)
return list_sum